Ng-model - How do I prevent all my checkboxes getting checked?

I sum up all my checkboxes inside my table with this 1 line:

<td><input class="checkbox" type="checkbox" name="dossier_id" value="" ng-model="master"></td>

When checking 1 of my checkboxes (via ng-model), all checkboxes get checked.

So my question is, how can I prevent this from happening?

PS: When working with ng-show, this is really disturbing.